1. Field of the Invention
The present invention relates to a transmission type liquid crystal display device which includes switching elements such as thin film transistors (hereinafter, referred to as "TFTs") as addressing elements and is used for displays of computers, TV sets, and the like, and a method for fabricating such a transmission type liquid crystal display device.
2. Description of the Related Art
FIG. 16 is a circuit diagram of a conventional transmission type liquid crystal display device provided with an active matrix substrate.
Referring to FIG. 16, the active matrix substrate includes a plurality of pixel electrodes 1 arranged in a matrix and TFTs 2 used as switching elements connected to the respective pixel electrodes 1. Gate electrodes of the TFTs 2 are connected to gate lines 3 for supplying a scanning (gate) signal, so that the gate signal can be input into the gate electrodes to control the driving of the TFTs 2. Source electrodes of the TFTs 2 are connected to source lines 4 for supplying an image (data) signal, so that the data signal can be input into the corresponding pixel electrodes 1 via the TFTs when the TFTs are being driven. The gate lines 3 and the source lines 4 run adjacent to the pixel electrodes 1 and are arranged in a matrix to cross each other. Drain electrodes of the TFTs 2 are connected to the respective pixel electrodes 1 and storage capacitors 5. Counter electrodes of the storage capacitors 5 are connected to common lines 6. The storage capacitor 5 is used for holding a voltage applied to a liquid crystal layer. The storage capacitor is provided in parallel to a liquid crystal capacitor which includes the liquid crystal layer sandwiched between a pixel electrode provided on an active matrix substrate and a counter electrode provided on a counter substrate.
FIG. 17 is a sectional view of a one-TFT portion of the active matrix substrate of the conventional liquid crystal display device.
Referring to FIG. 17, a gate electrode 12 connected to the gate line 3 shown in FIG. 16 is formed on a transparent insulating substrate 11. A gate insulating film 13 is formed covering the gate electrode 12. A semiconductor layer 14 is formed on the gate insulating film 13 so as to overlap the gate electrode 12 via the gate insulating film 13, and a channel protection layer 15 is formed on the center of the semiconductor layer 14. n.sup.+ -Si layers as a source electrode 16a and a drain electrode 16b are formed covering the end portions of the channel protection layer 15 and portions of the semiconductor layer 14, so that they are separated from each other at the top of the channel protection layer 15. A metal layer 17a which is to be the source line 4 shown in FIG. 16 is formed to overlap the source electrode 16a as one of the n.sup.+ -Si layers. A metal layer 17b is formed to overlap the drain electrode 16b as the other n.sup.+ -Si layer so as to connect the drain electrode 16b and the pixel electrode 1. An interlayer insulating film 18 is formed covering the TFT 2, the gate line 3, and the source line 4.
A transparent conductive film is formed on the interlayer insulating film 18 to constitute the pixel electrode 1. The transparent conductive film is connected to the metal layer 17b which is in contact with the drain electrode 16b of the TFT 2 via a contact hole 19 formed through the interlayer insulating film 18.
Thus, since the interlayer insulating film 18 is formed between the pixel electrode 1 and the underlying layers including the gate and source lines 3 and 4, it is possible to overlap the pixel electrode 1 with the lines 3 and 4. Such a structure is disclosed in Japanese Laid-Open Patent Publication No. 58-172685, for example. With this structure, the aperture ratio improves and, since the electric field generated by the lines 3 and 4 is shielded, the occurrence of disclination can be minimized.
Conventionally, the interlayer insulating film 18 is formed by depositing an inorganic material such as silicon nitride (SiN) to a thickness of about 500 nm by chemical vapor deposition (CVD).
The above conventional liquid crystal display device has disadvantages as follows.
When a transparent insulating film made of SiN.sub.x, SiO.sub.2, TaO.sub.x, and the like is formed on the interlayer insulating film 18 by CVD or sputtering, the surface of the film directly reflects the surface profile of the underlying film, i.e., the interlayer insulating film 18. Therefore, when the pixel electrode 1 is formed on the transparent insulating film, steps will be formed on the pixel electrode 1 if the underlying film has steps, causing disturbance in the orientation of liquid crystal molecules. Alternatively, the interlayer insulating film 18 may be formed by applying an organic material such as polyimide to obtain a flat pixel portion. In such a case, however, in order to form the contact holes for electrically connecting the pixel electrodes and the drain electrodes, a series of steps including photopatterning using a photoresist as a mask, etching for forming the contact holes, and removal of the photoresist are required. A photosensitive polyimide film may be used to shorten the etching and removal steps. In this case, however, the resultant interlayer insulating film 18 appears colored. This is not suitable for a liquid crystal display device requiring high light transmission and transparency.
The other disadvantage is as follows. When the pixel electrode 1 overlaps the gate line 3 and the source line 4 via the interlayer insulating film 18, the capacitances between the pixel electrode 1 and the gate line 3 and between the pixel electrode 1 and the source line 4 increase. In particular, when an inorganic film made of silicon nitride and the like is used as the interlayer insulating film 18, the dielectric constant of such a material is as high as 8 and, since the film is formed by CVD, the thickness of the resultant film is as small as about 500 nm. With such a thin interlayer insulating film, the capacitances between the pixel electrode 1 and the lines 3 and 4 are large. This causes the following problems (1) and (2). Incidentally, in order to obtain a thicker inorganic film made of silicon nitride and the like, an undesirably long time is required in the aspect of the fabrication process.
(1) When the pixel electrode 1 overlaps the source line 4, the capacitance between the pixel electrode 1 and the source line 4 becomes large. This increases the signal transmittance, and thus a data signal held in the pixel electrode 1 during a holding period fluctuates depending on the potential thereof. As a result, the effective voltage applied to the liquid crystal in the pixel varies, causing, in particular, vertical crosstalk toward a pixel adjacent in the vertical direction in the actual display.
In order to reduce the influence of the capacitance between the pixel electrode 1 and the source line 4 appearing on the display, Japanese Laid-Open Patent Publication No. 6-230422 proposes a driving method where the polarity of a data signal to be supplied to the pixels is inverted every source line. This driving method is effective for a black-and-white display panel where the displays (i.e., data signals) of adjacent pixels are highly correlated with each other. However, it is not effective for a color display panel for normal notebook type personal computers and the like where pixel electrodes are arranged in a vertical stripe shape (in color display, a square pixel is divided into three vertically long rectangular picture elements representing R, G, and B, forming a vertical stripe shape). The display color of pixels connected to one source line is different from that of pixels connected to an adjacent source line. Accordingly, the proposed driving method of inverting the polarity of the data signal every source line is not effective in reducing crosstalk for the general color display, though it is effective for the black-and-white display.
(2) When the pixel electrode 1 overlaps the gate line 3 for driving the pixel, the capacitance between the pixel electrode 1 and the gate line 3 becomes large, increasing the feedthrough of the write voltage to the pixel due to a switching signal for controlling the TFT 2.